The Effect of Diaphragms in Prestressed Concrete

16. Abstract Four scale microconcrete model bridges were tested. Experimental variables included span, skew angle, stiffness, location and number of diaphragms. Service load level behavior was studied under static, cyclic and impact loads with successive removal of diaphragms. Behavior at overload and ultimate load conditions was documented from ultimate static and impact load tests. Characteristics studied were load distribution under static service loads and overloads, ultimate load capacities and failure modes of girders, cracking and ultimate load capacities of slabs, dynamic amplification, bridge damping, fundamental modes of vibration and natural frequencies, response to lateral impacts, and stresses in diaphragms. A computer program was verified with experimental results and used to generalize findings. The only significant role of interior diaphragms found is to distribute the load more evenly. In no case was an appreciable reduction in the governing design moment found. A cost analysis showed it would be more economical to provide increased girder strength than to rely on improved distribution of load decreasing the girder design moment due to provision of diaphragms. Design live loads for girders determined from the distribution factors of the 1969 AASHO specifications were found to be conservative even without diaphragms. Provision of interior diaphragms made the girders more vulnerable to damages from lateral impacts. It was recommended that interior diaphragms not be provided in simply supported prestressed concrete girder and slab bridges. Provision of exterior diaphragms or some alternate method of supporting or strengthening the free edge of the transverse slab was considered necessary for reliable serviceability.